The present invention relates to coating or modifying a substrate using corona-generated chemical vapor deposition.
The use of widely available and inexpensive polymers such as polyolefins is often limited by the undesirably low surface energy of these polymers. Consequently, more expensive materials having higher surface energy are often used where surface wettability or adhesion or both, are required. In recent years, an alternative approach has been developed, namely surface modification of low surface energy polymers using corona or plasma discharge.
For example, U.S. Pat. No. 5,576,076 (Slootman et al.) teaches that the performance of polyolefin film can be improved by creating a deposit of silicon oxide on a traveling substrate by subjecting the substrate to a corona discharge at atmospheric pressure in the presence of a silane such as SiH4, a carrier gas, and oxygen or a gas capable of producing oxygen. Although the method described by Slootman et al. does indeed render the surface of the polymer more wettable, it suffers from at least two drawbacks. First, the preferred working gas (SiH4) is an extremely hazardous material that ignites spontaneously in air; second, the deposition of silicon oxide is in the form of a powder, the creation of which limits the scope of potential applications and which rapidly fouls up equipment resulting in substantial down time.
Glow discharge plasma enhanced chemical vapor deposition (PECVD) has been used to produce coatings on substrates to improve their resistance to chemicals, wear, abrasion, scratching, and gas permeation. For example, in U.S. Pat. No. 6,106,659, Spence, et al. describes a cylinder-sleeve electrode assembly apparatus that generates plasma discharges in either an RF resonant excitation mode or a pulsed voltage excitation mode. The apparatus is operated in a rough-vacuum mode, with working gas pressures ranging from about 10 to about 760 Torr. Operation at rough-vacuum pressure is said to have advantages over operation at strictly atmospheric pressure because the required supply gas flow rate is significantly reduced compared to strictly atmospheric operation, allowing for the economical use of more expensive specialty gases. Furthermore, the generated coatings possess superior properties as compared to coatings formed using conventional corona-type discharge systems operating either at low or high pressures.
The method described by Spence, et al. suffers from the requirement of rough vacuum, which is a commercial disadvantage over strict atmospheric methods. Thus, it would be an advantage in the art of PECVD to be able to create contiguous (that is, non-powder-forming) coatings at atmospheric pressure.
The present invention addresses the deficiencies in the art by providing a process for preparing an optically clear deposit onto a substrate comprising the steps of 1) creating a corona discharge in a region between a) an electrode having at least one inlet and at least one outlet and b) a counterelectrode supporting a substrate; and 2) flowing a mixture of a balance gas and a working gas and, optionally, a carrier gas for the working gas through the electrode and the corona discharge at a sufficient flow rate and at such proportions to form the optically clear deposit onto the substrate.
In a second aspect, the present invention is a process for preparing a deposit onto a substrate comprising the steps of 1) creating a corona discharge in a region between a) an electrode having at least one inlet and at least one outlet and b) a counterelectrode supporting a substrate; and 2) flowing a mixture of a balance gas and a working gas and, optionally, a carrier gas for the working gas through the electrode and the corona discharge so as to form a plasma polymerized deposit on the substrate, wherein the total gas mixture has a flow rate such that the velocity through the at least one outlet is not less than 0.1 m/s and not greater than 1000 m/s and wherein the concentration of working gas based on the total gas mixture is not less than 5 ppm and not greater than 500 ppm.
In a third aspect, the present invention is a continuous process for preparing an optically clear coating onto a moving substrate comprising the steps of 1) creating a corona discharge in a region between a) an electrode having at least one inlet and at least one outlet and b) a counterelectrode supporting a moving substrate; and 2) flowing a mixture of a balance gas, a working gas and, optionally, a carrier gas for the working gas through the electrode and the corona discharge so as to form a plasma polymerizing coating on the substrate, wherein the balance gas has a flow rate such that the velocity through the at least one outlet is not less than 10 m/s and not greater than 200 m/s, wherein the concentration of the working gas based on the total gas mixture is not less than 5 ppm and not greater than 200 ppm, wherein the optically clear coating has an optical clarity of at least 98 percent and a haze value of not greater than 2 percent.